The present invention is directed to ink-jet printers and in particular to mechanisms for controlling the ink duty.
A digitally expressed color image is quite often stored or expressed as three-dimensional values of picture elements ("pixels") that in combination make up the image. The pixel value represents a location in a color space. Most typically, that color space is the red-green-blue color space or, complementarily, the cyan-magenta-yellow color space. That is, the vector components respectively represent red, green, and blue or cyan, magenta, and yellow. Other color spaces are used less frequently.
Most imaging systems express the pixel-value components with a relatively high resolution, such as eight bits per pixel, at some point in the processing. In such a case, each component can have a value between 0 and 255 (2.sup.8 -1=255). In contrast, although some printers do apply ink dots of different sizes, most apply only single-sized dots, so the control for a given color component is binary: the dot is either applied or not at any given pixel location. The higher intensity resolution of the original pixel values is simulated by half-toning, in which different intensities are achieved by applying dots in different percentages of the pixels. If the component-value range is 255, and the value of a particular component is 255 throughout a region, then all pixels in that region receive a dot of the ink whose color is associated with that component. If the component value is 128, on the other hand, only 128 out of every 255 pixels receive ink of that color.
In "ordered dither"-type half-toning, for instance, thresholds of a "dither matrix" are associated with respective display-medium pixel locations, and the printer deposits ink on those locations at which the component value equals or exceeds the associated matrix threshold. So if the matrix consists of threshold values more or less evenly distributed throughout a range, a component value in the middle of the range is likely to result in dots' being deposited at approximately half of the pixel locations, whereas a component value at the top of the range is deposited in all locations: the ink duty is 100%.
The form in which the image typically is stored electronically represents quantities of ideal color components, which real-world inks do not match exactly. Moreover, ink-dot geometry is such that color darkness does not increase linearly with the ink's duty cycle. So it is a feature of some printer drivers that they adjust the image-representing signals to compensate for these limitations and produce signals that specify the ink duties that would match the ideal image with the greatest fidelity.
But the medium on which the image is printed unfortunately imposes further limitations in some cases. In particular, the high ink duties that would produce the darkest requested colors most faithfully would cause ink "bleeding" on some types of paper and other media. Coated types of paper are typically subject to such bleeding, and transparencies often are similarly afflicted. So the printer driver must additionally impose an ink-duty limit, which necessarily compromises image fidelity. This compromise is unavoidable: there is no way in which a limited ink duty can result in as faithful a reproduction of the darkest colors as ink duties can that are not so limited. So it would seem that the best that can be done for unadjusted ink duties that exceed a given medium's ink-duty limit is to print them with the limit value and thus obtain a darkness as great as the medium's limitations permit.